Investment casting, also known as lost wax, lost pattern or precision casting, is a process employed in a number of industries to make metal, glass, and ceramic articles that meet relatively close dimensional tolerances. Typically, an investment casting is made by first making a facsimile or pattern from a meltable substrate of the object to be made by investment casting. Suitable meltable substrates may include, for example, wax, polystyrene, or plastic.
Next, a ceramic mold, known as an investment casting shell, is formed around the pattern. This process may include dipping the pattern into a slurry containing a mixture of liquid refractory binders and a refractory powder and then sieving dry refractory grains onto the freshly dipped pattern. The most commonly used dry refractory grains include quartz, fused silica, zircon, alumina and aluminosilicate. The steps of dipping the pattern into a refractory slurry and then sieving dry refractory grains onto the freshly dipped pattern may be repeated until the mold has sufficient thickness and strength for further processing. However, it is preferable if each coat of slurry and refractory grains is air-dried before subsequent coats are applied. The investment slurry is then given time to set and dry. Drying can be accelerated by forced air and other techniques.
After drying, the shell is heated to at least the melting point of the meltable substrate. The heat melts the substrate away, leaving only the shell and possibly some residual substrate. The shell may be heated to a temperature high enough to vaporize any residual meltable substrate from the shell. Generally, the shell is filled with molten metal before the shell has cooled from the high temperature heating. Various methods have been used to introduce molten metal into shells including gravity, pressure, vacuum and centrifugal methods. When the molten metal in the mold has solidified and cooled sufficiently, the casting may be removed from the shell.
Although investment casting has been known and used for many years, the investment casting market continues to grow as the demand for more intricate and complicated parts increase. Because of the great demand for high quality, precision castings, there continuously remains a need to develop new ways to make investment casting shells more quickly, efficiently, cheaply and of higher quality. For instance, if the strength of investment casting shells could be increased, less material would be required. If an investment casting shell could be made with fewer coatings, it could be made more quickly, resulting in time and cost savings.